MetaMask is an extension for accessing Ethereum enabled distributed applications, or "Dapps" in your normal browser. The extension injects the Ethereum web3 API into every website's javascript context, so that dapps can read from the blockchain. MetaMask requires no login and does not store your private keys in any server, instead they are stored on Chrome and password protected.
Official Explainer Video
MetaMask Details
by ConsenSys AG
New York, USA
Ethereum